1. Field of the Invention
The present invention relates to a pressure vibration damping device for incorporation in a liquid supply passage for supplying a liquid from a pump to a hydraulic device.
2. Description of the Related Art
In a hydraulic circuit of a hydraulic device such as a fluid pressure type suspension in a vehicle such as an automobile that requires a constant supply of a liquid at a high pressure, there is generally incorporated a high pressure pump which is excellent in durability but generates relatively large pulses in the flow rate of the liquid. In the hydraulic circuit incorporating such a high pressure pump, relatively large pulses in the flow rate of the liquid are generated due to the liquid flow pulses generated by the pump. Those pulses can not be sufficiently damped by a simple and convenient pressure vibration damping device such as disclosed in, for example, Japanese Utility Model Laid-open Publication 61-110476. Therefore, in a fluid circuit such as the hydraulic circuit of the fluid pressure type suspension it is required that a pressure pulse absorbing means having a higher pressure pulse absorbing performance be incorporated.
However, it was found that when a pressure pulse absorbing means having a high pressure pulse absorbing performance is incorporated in a liquid supply passage, even a larger pressure vibration is generated at a portion closed to the outlet port of the pump in a particular rotational region of the pump, said large pressure vibration having an amplitude 4 to 5 times as large as that generated when no such pressure pulse absorbing means is incorporated, probably because of a resonance between the pulsating output pressure and the pressure pulse absorbing performance of the pressure pulse absorbing means. This amplified pressure vibration was found as being caused by a liquid column vibration having a cycle period corresponding to a cycle period of pulses of the pump that is, for example, a period during which the piston of the pump rotates for one pitch. FIGS. 9 and 10 show the changes in the flow rate and in the pressure of the liquid, respectively, which are observed when such a liquid column vibration occurs. In these figures, the solid lines show those changes in the vicinity of the outlet port of the pump, and the broken lines show those changes in the vicinity of the inlet of the pressure pulse absorbing means. Since no such liquid column vibration occurs in a liquid supply passage incorporating no such pressure pulse absorbing means, it is contemplated that such a liquid column vibration is more apt to occur as the pulse absorbing performance of the pressure pulse absorbing means is improved.
Further, since the region of the rotational speed of the pump in which the liquid column vibration occurs is in the region of the normal rotational speed of the pump, it is not possible to avoid the generation of the liquid column vibration in the liquid supply passage incorporating the pressure pulse absorbing means by shifting the operational speed region of the pump. When the liquid column vibration occurs in the hydraulic system in a vehicle, unpleasant noises and/or vibrations of the vehicle body occur, deteriorating the comfortableness of the vehicle and the durability of the pump and the conduits in the hydraulic circuit.
As a result of various research on the above-mentioned liquid column vibration, the inventors have found that when there exists a certain condition with respect to the phase relation between the vibrating pressures at an upstream end portion and a downstream end portion of a liquid supply passage, said condition being such that, for example, the phase difference between the vibrating pressures at the upstream end portion and the downstream end portion of the liquid supply passage is 180 degrees, there occurs a large liquid column vibration so that the flow rate of the liquid is momentarily so much increased as to generate a pulse, and that such a liquid column vibration can be effectively damped by providing a certain flow resistance in the passage in accordance with the vibration of flow in the passage.